Add modern UI enhancements over the classic redesign

Layered progressive polish on the warm classic + brass theme:
- Card entrance animation on first mount (virtualizer-aware)
- Cursor-tracking brass spotlight border on cards
- Thumbnail skeleton shimmer until the poster paints
- Hover video preview after a short dwell (only when formats resolved)
- View Transition + blurred-poster ambient backdrop on player open
- Favorite heart pop + expanding ring on add
- ⌘K command palette (search, theme, density, reels, source/channel)
- Scroll-progress bar + back-to-top FAB
- Grid density toggle (comfortable/compact)
- Reels HUD: serif title, brass scrubber, muted-state pulse

All new motion respects prefers-reduced-motion; no JS/HTML structure
changes to the core grid/feed. New glue lives in frontend/js/enhance.js.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/favorites.js

@@ -77,6 +77,7 @@ App.favorites = App.favorites || {};
         if (!key) return;
         const favorites = App.favorites.getAll();
         const existingIndex = favorites.findIndex((item) => item.key === key);
+        const becameFavorite = existingIndex < 0;
         if (existingIndex >= 0) {
             favorites.splice(existingIndex, 1);
         } else {
@@ -86,6 +87,15 @@ App.favorites = App.favorites || {};
         App.favorites.setAll(favorites);
         App.favorites.renderBar();
         App.favorites.syncButtons();
+        // Celebrate an add with a brass pop + ring on every button for this key.
+        if (becameFavorite) {
+            document.querySelectorAll(`.favorite-btn[data-fav-key="${(window.CSS && CSS.escape) ? CSS.escape(key) : key}"]`).forEach((btn) => {
+                btn.classList.remove('just-favorited');
+                void btn.offsetWidth; // restart the animation
+                btn.classList.add('just-favorited');
+                btn.addEventListener('animationend', () => btn.classList.remove('just-favorited'), { once: true });
+            });
+        }
     };
 
     App.favorites.renderBar = function() {